Well-known is a safety helmet, such as jet-type one, semi-jet-type one or full-face-type one, which includes a head protector (hereinafter described in this text only as "head covering"), which is used to protect the head of a man with a helmet on, such as a rider on a motor-bicycle ("rider" is hereinafter used to mean "a rider or other user"). The conventional jet-type, semi-jet-type, or full-face-type helmet has generally a head covering and a pair of right and left chin straps, which are secured on the inside of the head covering, and typically constituted as follows.
The head covering has a cut (in the case of the jet-type or semi-jet-type helmet) or an opening (in the case of the full-face-type helmet), which is formed in the front of a part between the forehead and the chin (that is, the face). The jet-type or the semi-jet-type helmet further has a visor, which is secured to the head covering adjacent to the upper edge of the recess. The full-face-type helmet further has a shield plate, which is secured to the head covering so as to be movable between a lower position, where it closes the opening, and a upper position, where it opens the opening. The shield plate can be used for the jet-type helmet and the semi-jet-type helmet, for example in lieu of the visor. In that event, the shield plate can open and close the recess.
The head covering comprises an outer shell, which forms the outer peripheral wall of the head covering; a rim member; and a backing member, which is brought into contact with the inner surface of the outer shell and fixed by adhesive or so. The rim member is fixed to the rim of the outer shell by adhesive so that all the rim of the outer shell (in the case of the full-face-type helmet, all the rim of the opening is also included) is put between the rim member. The backing member includes a backing member for the head, which is to face the sinciput, the vertex, the temples and the occiput. The backing member of the jet-type or the semi-jet-type helmet further includes two backing members for the two ears, each of which is to face the ears, or the backing member for the head is integral with backing members for the ears. The backing member of the full-face-type helmet further includes a backing member for the chin, which is to face to the chin.
The backing member for the head comprises an impact-on-the-head absorbing liner and an air permeable back cover. The back cover for the head is secured to the impact absorbing liner by an adhesive or a tape so that the back cover for the head covers the inner surface of the impact absorbing liner (sometimes, an area to face the vertex of the rider or the like's head is partially excluded), the side surface (that is, a narrow surface lying between the inner surface and the outer surface), and the periphery of the outer surface which extends from the side surface. The impact absorbing liner is made of foamed synthetic resin, such as polystylene, polypropylene or polyethylene. The backing member for the jaw also has substantially the same structure as that of the backing member for the head except for having such a shape as to correspond to the rider's jaw. If necessary, a pair of right and left block-like inner pads is attached to a part of the inner surface of the impact-on-the-jaw absorbing liner (for example, two areas that face the rider's cheeks). Thus, this block-like inner pad is positioned between the impact-on-the-jaw absorbing liner and the back cover for the jaw. The backing member for each ear also has substantially the same structure as that of the backing member for the head or jaw except for having such a shape as to correspond to the rider's ear.
When impact is exerted on a part of the outer shell of the conventional safety helmet typically constituted as above, the impact is widely dispersed throughout the outer shell and the impact energy is absorbed by the deformed outer shell. Further, the impact absorbing liner functions in absorbing the impact energy propagated from the outer shell by means of its deformation, absorbing the impact energy by means of its thickness reduction (that is, compressive deformation), and lowering the maximum acceleration due to the impact by means of delaying the propagation of this impact energy exerted on the rider's head. In this text, the "maximum acceleration" means the maximum value of the acceleration through an impact absorbing test for the helmet.
Up to nowadays, the above impact absorbing test has been made to ascertain the protective characteristic of the safety helmet. In this impact absorbing test, a metallic head model imitating the head of a man with a helmet on is used, in which an accelerometer is incorporated. A standard on the maximum acceleration measured with the accelerometer has been adopted in each country. Further, an index of HIC (Head Injury Criteria) has been proposed on the basis of correlation between the mean acceleration value of a certain duration and the duration of the appearances of values continuously over this mean acceleration value, and the brain damage. HIC is given by: ##EQU1## where a(t) is the change of acceleration value with time during the impact absorbing test, and t.sub.1, t.sub.2 are the time, each maximizing the HIC value.
The HIC value has been considered to have good correlation with the level of the injuries suffered in an accident. According to Mr. P. D. Hope of Transport and Road Research Laboratory established in England, in an accident on the motor-bicycle, when the HIC value is 1,000, the probability of mortality is 8.5%; when the HIC value is 2,000, the probability of mortality is 31%; and the HIC value is 4,000, the probability of mortality is 65%. Therefore, it is necessary to lower the HIC value in order to lower the level of the injuries.
As described above, it is necessary to lower the maximum acceleration value and the HIC value due to the impact, if it is desired to improve the protective characteristic of the safety helmet. Such being the case, in order to lower the maximum acceleration value and the HIC value, the thickness of the impact absorbing liner has been increased so far.
However, only to increase the thickness of the impact absorbing liner is insufficient for the reduction of the maximum acceleration value and, particularly, difficult for the reduction of the HIC value. The HIC value includes the duration of the appearances of values continuously over a specific acceleration value, so that even if the maximum acceleration value is a little lowered due to a cushion characteristic of the impact absorbing liner, it is impossible to shorten the duration of the appearances of values continuously over the specific acceleration value. Thus, it is impossible to reduce the HIC value.